Screw pump and screw gear

ABSTRACT

A screw pump is provided with a pair of screw rotors serving as fluid transfer bodies. With respect to a rotation angle (x) around an axis of each of the screw rotors, a change of a lead angle (θ) from a winding start angle ( 0 ), which is the rotation angle (x) corresponding to a leading end of a spiral groove, to a winding end angle (E), which is the rotation angle (x) corresponding to a trailing end of the spiral groove, can be expressed by a lead angle change function θ(x). The lead angle change function θ(x) is structured by a combination of a plurality of change functions θ 1 (x) and θ 2 (x) having different manners of changing. It is possible to arbitrarily set a manner in which the lead angle (θ) changes in accordance with a combination of a plurality of change functions θ 1 (x) and θ 2 (x). Therefore, it is possible to arbitrarily set a fluid compression characteristic of the pump in a relation with an axial length (L) of the screw rotor.

TECHNICAL FIELD

The present invention relates to a screw pump, for example, used in asemiconductor manufacturing process and a screw gear suitable for theuse in a screw pump.

BACKGROUND ART

In general, in a semiconductor manufacturing process, a screw pump isused as a vacuum pump for generating a vacuum environment. In otherwords, in the semiconductor manufacturing process, in order to applyvarious processes to a wafer under a vacuum environment, a clean vacuumenvironment is generated within a container by supplying an inert gassuch as F₂ gas or the like, which is a fluid, into a container in whichthe wafer is accommodated, and the gas is sucked together with animpurity (O₂, CO₂ or the like) remaining within the container by avacuum pump. As such a vacuum pump, there has been conventionally knowna screw pump, for example, described in Patent Document 1.

The screw pump described in the Patent Document 1 is configured suchthat a pair of screw gears spirally mating with each other, that is, apair of screw rotors serve as fluid transfer bodies (a gas transferbodies). Each of the screw gears is coupled to a rotary shaft rotated bya drive source so as to be integrally rotated. A lead angle (a torsionangle) of each of the screw gears is continuously changed in accordancewith a spiral groove (a helix) of the screw gear. Specifically, the leadangle is monotonically increased toward an end portion in an axialdirection close to a high pressure (discharge) side from an end portionin an axial direction close to a low pressure (intake) side in the screwgear. In this case, the lead angle is defined as an angle of slope ofthe spiral groove with respect to an axis of the screw gear. In the casethat both the screw gears are rotated in accordance with the rotation ofthe rotary shafts, the inert gas is sucked into a pump chamber from theoutside, is transferred to the discharge side while being compressed byboth the screw gears within the pump chamber, and is thereafterdischarged to the outside from the interior of the pump chamber.

FIG. 4A is a graph showing a manner in which a lead angle θ changes inthe screw gear in the Patent Document 1. FIG. 4A shows a change of thelead angle θ from a leading end (an intake side end portion) of thespiral groove (the helix) of the screw gear to a trailing end (adischarge side end portion) by setting a rotation angle x of the spiralgear around an axis to a horizontal axis. As shown in FIG. 4A, thechange of the lead angle θ in the spiral groove from the intake side endportion to the discharge side end portion can be expressed as a functionθ(x) of the rotation angle x of the screw gear around the axis. Withrespect to the horizontal axis of the graph in FIG. 4A, the rotationangle x corresponding to the intake side end portion of the spiralgroove is defined as a winding start angle 0, and the rotation angle xcorresponding to the discharge side end portion of the spiral groove isdefined as a winding end angle E.

As shown in the graph in FIG. 4A, the lead angle θ is monotonicallyincreased from a winding start lead angle DegS (for example, 50 degrees)serving as the lead angle corresponding to the winding start angle 0, toa winding end lead angle DegE (for example, 80 degrees) serving as thelead angle corresponding to the winding end angle E. Accordingly, in thePatent Document 1, as shown in FIG. 4B, an entire length L in the axialdirection of the screw gear is univocally defined by a monotoneincreasing function θ(x) using the winding start lead angle DegS and thewinding end lead angle DegE.

In other words, the monotone increasing function θ(x) indicating thechange of the lead angle θ of the screw gear can be expressed by thefollowing expression (11), and a constant k in the expression (11) canbe expressed by the following expression (12). In this case, referencesymbol r denotes a radius of a pitch circle of the screw gear.

θ(x)=DegS+k·x  (11)

k=(DegE−DegS)/(2πr·E)  (12)

In accordance with the expressions (11) and (12), the entire length L ofthe screw gear can be univocally defined by the following expression(13).

L=1/k·log(sin(DegS+k·2πr·E)/Sin(DegS))  (13)

The expression (13) mentioned above indicates the fact that the entirelength L of the screw gear is determined by the winding start lead angleDegS and the winding end lead angle DegE in the screw gear.

Further, in the screw pump of the Patent Document 1 mentioned above, avolumetric capacity of a plurality of gas actuation chambers definedwithin the pump chamber by the screw gear becomes gradually small towardthe discharge side from the intake side, and the gas is compressed as itis transferred toward the discharge side actuation chamber. To changethe manner in which the volumetric capacity of the actuation chamber ischanged from the intake side to the discharge side, in other words, tochange the manner in which a gas compression characteristic of the screwpump, the winding start lead angle DegS and the winding end lead angleDegE affecting the entire length L of the screw gear are changed. On theother hand, since the screw gear is accommodated within the pump chamberin the vacuum pump, it is necessary that the entire length L of thescrew gear be set to a value by which the screw gear can be accommodatedwithin the pump chamber. However, in the case of changing the windingstart lead angle DegS and the winding end lead angle DegE for changingthe gas compression characteristic of the screw pump, there can begenerated a case that the entire length L of the screw gear comes to avalue by which the screw gear cannot be accommodated within the pumpchamber. Accordingly, the gas compression characteristic of the screwpump in the Patent Document 1 cannot be freely adjusted.

Patent Document 1: Japanese Laid-Open Patent Publication No. 9-32766DISCLOSURE OF THE INVENTION

An objective of the present invention is to provide a screw pump and ascrew gear which have a high degree of flexibility in a fluidcompression characteristic.

In order to achieve the objective mentioned above, in accordance withthe present invention, there is provided a screw gear having a portionin which a lead angle is continuously changed from a leading end to atrailing end of a spiral groove. In the case of expressing, as a leadangle change function, a change of the lead angle from a winding startangle serving as a rotation angle corresponding to the leading end ofthe spiral groove to a winding end angle serving as a rotation anglecorresponding to a trailing end of the spiral groove with respect to arotation angle of the screw gear around an axis, the lead angle changefunction is constituted by a combination of a plurality of changefunctions having different manners of changing.

Further, in accordance with the present invention, there is provided ascrew pump apparatus comprising a pair of screw gears mating with eachother and a pump chamber accommodating the screw gears. The screw gearsrotate while mating with each other, whereby a fluid sucked into thepump chamber is transferred in an axial direction of the screw gearswhile being compressed within the pump chamber. Each of the screw gearsis constituted by the screw gear structured as mentioned above, and anactuation chamber for compressing the fluid is defined between adjacentthread ridge portions in the axial direction of each respective screwgear.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional plan view of a screw vacuum pump inaccordance with an embodiment of the present invention;

FIG. 2A is a graph showing a change of a lead angle of a screw rotor;

FIG. 2B is a graph explaining an axial length of the screw rotor;

FIG. 3A is a graph showing a change of a lead angle of a screw rotor;

FIG. 3B is a graph explaining an axial length of the screw rotor;

FIG. 4A is a graph showing a change of a lead angle of a prior art screwrotor; and

FIG. 4B is a graph explaining an axial length of the prior art screwrotor.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will be given of one embodiment according to the presentinvention with reference to FIGS. 1 to 3B.

As shown in FIG. 1, a screw vacuum pump 11 in accordance with thepresent embodiment is provided with a cylindrical rotor housing member12, a lid-shaped front housing member 13 jointed to a front end (a leftend in FIG. 1) of the rotor housing member 12, and a plate-shaped rearhousing member 14 joined to a rear end (a right end in FIG. 1) of therotor housing member 12. A stepped mounting hole 14 a is formed in therear housing member 14, and a bearing body 15 is fixed to the rearhousing member 14 by bolts in a state of being fitted to the mountinghole 14 a. A pair of screw rotors (screw gears) 16 serving as fluidtransfer bodies are accommodated within the rotor housing member 12. Apump chamber 17 is defined between outer peripheral surfaces of thescrew rotors 16 and an inner peripheral surface of the rotor housingmember 12. The specific structure of the screw rotors 16 will bedescribed later.

A pair of support holes 18 are formed in the bearing body 15 to extendtherethrough, and a rotary shaft 19 is inserted in and supported by eachof the support holes 18. An end portion (a left end in FIG. 1) of eachof the rotary shafts 19 protrudes into the pump chamber 17 from thecorresponding support hole 18, and one of both the screw rotors 16 isfixed to the end portion of each of the rotary shafts 19 by a bolt. Inother words, each of the screw rotors 16 is coupled to the correspondingrotary shaft 19 so as to integrally rotate with the rotary shaft 19.

A gear housing member 20 formed as a cylinder having a closed end isfixed to a rear end of the rear housing member 14. End portions (rightends in FIG. 1) 19 a of both the rotary shafts 19 respectively protrudeinto the gear housing member 20, and gears 21 are fastened and attachedto the protruding end portions 19 a in a mating state. An electric motor22 serving as a drive source is attached to an outer surface of the gearhousing member 20. The end portion 19 a of one rotary shaft (the rotaryshaft in a lower side in FIG. 1) 19 of both the rotary shafts 19 iscoupled to an output shaft 22 a of the electric motor 22 extending intothe gear housing member 20 via a shaft coupling 23.

An suction port 24 allowing a fluid, specifically an inert gas such asF₂ gas or the like to be introduced is formed substantially in a centerportion of the front housing member 13 in such a manner as to becommunicated with the pump chamber 17. A discharge port (not shown)allowing the inert gas to be discharged is formed in a peripheral wallof the rotor housing member 12 near an end portion of the rotor housingmember 12 positioned in an opposite side to the suction port 24 in sucha manner as to be communicated with the pump chamber 17. The dischargeport is positioned in a lower portion in an substantially center of awidth direction (a vertical direction in FIG. 1) of the rotor housingmember 12. If the electric motor 22 is driven, the screw rotors 16 arerotated inversely in accordance with the rotation of the rotary shafts19. Accordingly, the inert gas sucked into the pump chamber 17 via thesuction port 24 is transferred toward the discharge port within the pumpchamber 17 in an axial direction of the screw rotors 16 while beingcompressed, and is thereafter discharged to an outer portion from thedischarge port.

Next, a description will be given of the screw rotors 16.

As shown in FIG. 1, each of the screw rotors 16 is formed as a singlethread screw gear, and has a spiral groove, that is, a thread ridge 16 aand a thread groove 16 b on an outer peripheral surface thereof. Thescrew rotors 16 extend in parallel to each other within the pump chamber17 such that the thread ridge 16 a on one of the screw rotors 16 and thethread groove 16 b in the other are mated with each other. Actuationchambers 25 for the inert gas are formed between the adjacent threadridges 16 a in the axial direction of the respective screw rotors 16within the pump chamber 17. These actuation chambers 25 transfer theinert gas toward the discharge port from the suction port 24, that is,toward a high pressure side from a low pressure side, while compressingthe inert gas.

Each of the screw rotors 16 has a lead angle (also called as a torsionangle) θ continuously changing in accordance with a spiral groove (ahelix) of the screw rotor 16. In this case, the lead angle θ is definedas an angle of slope of the spiral groove (the thread ridge 16 a and thethread groove 16 b) with respect to an axis of the screw rotor 16. Thescrew rotor 16 is formed such that a lead P1 in a portion closest to thesuction port 24 becomes maximum and a lead P4 in a portion closest tothe discharge port becomes minimum, so that a volumetric capacity of theactuation chamber 25 is gradually reduced from the suction port 24 (theintake side) toward the discharge port (the discharge side).Specifically, the lead angle θ is changed such that the lead graduallybecomes smaller from a maximum lead P1 to a smaller lead P2 in a firstrange (an intake side range) from the portion closest to the suctionport 24 of the screw rotor 16 to an intermediate position m in the axialdirection. The lead angle θ is changed in a different manner from thatin the first range, such that the lead gradually becomes shorter from alead P3 to a smaller lead P4 in a second region (a discharged siderange) from the intermediate position m of the screw rotor 16 to theportion closest to the discharge port. In the present embodiment, sincethe screw rotor 16 is formed as the shape of the single thread screwgear, a distance in the axial direction at a time of making one turn ofthe axis of the screw rotor 16 along the lead of the screw rotor 16,that is, the spiral groove (the helix) is equal to the pitch of thethread ridge 16 a.

FIG. 2A is a graph showing a manner in which the lead angle θ of thescrew rotor 16 changes in the present embodiment. FIG. 2A, in which thehorizontal axis represents the rotation angle x around the axis of thescrew rotor 16, shows the change of the lead angle θ from a leading end(an intake side end portion) to a trailing end (a discharge side endportion) of the spiral groove (the helix) of the screw rotor 16. Asshown in FIG. 2A, the change of the lead angle θ in the spiral groovefrom the intake side end portion to the discharge side end portion canbe expressed as a function θ(x) of the rotation angle x around the axisof the screw rotor 16. Hereinafter, the function θ(x) is called as thelead angle change function θ(x). In this case, with respect to thehorizontal axis of the graph in FIG. 2A, the rotation angle xcorresponding to the intake side end portion of the spiral groove isdefined as a winding start angle 0, the rotation angle x correspondingto the intermediate position m is defined as a switch angle M, and therotation angle x corresponding to the discharge side end portion of thespiral groove is defined as a winding end angle E. In other words, inthe case of tracking back the spiral groove from the intake side endportion to the discharge side end portion while turning around the axisof the screw rotor 16, the rotation angle x corresponding to the intakeside end portion of the spiral groove is defined as the winding startangle 0, the rotation angle x at a time of reaching the intermediateposition m is defined as the switch angle M, and the rotation angle x ata time of reaching the discharge side end portion of the spiral grooveis defined as the winding end angle E.

As shown in FIG. 2A, during a period of the rotation angle x from thewinding start angle 0 to the winding end angle E, the lead angle changefunction θ(x) is constituted by a combination of a plurality of (two inFIG. 2A) change functions θ1(x) and θ2(x) having different manners ofchanging. In other words, during a period of the rotation angle x fromthe winding start angle 0 to the winding end angle E, the change of thelead angle θ is expressed by the combination of a plurality of changefunctions θ1(x) and θ2(x) having different manners of changing.

The change function θ1(x) corresponds to a first change function (anintake side change function) corresponding to an angle range from thewinding start angle 0 to the switch angle M, and expresses the change ofthe lead angle θ in the first range (the intake side range). The changefunction θ2(x) corresponds to a second change function (the intake sidechange function) corresponding to an angle range from the switch angle Mto the winding end angle E, and expresses the change of the lead angle θin the second range (the discharge side range). The second changefunction θ2(x) expresses the change of the lead angle θ by a slow changedegree in comparison with the first change function θ1(x). Both of thefirst change function θ1(x) and the second change function θ2(x) areconstituted by a monotone increasing function which gradually increasesthe lead angle θ in accordance with change of the rotation angle x fromthe winding start angle 0 toward the winding end angle E.

With respect to a vertical axis of the graph in FIG. 2A, “DegS”corresponds to a lead angle in the intake side end portion of the spiralgroove corresponding to the winding start angle 0, that is, a windingstart lead angle, “DegM” corresponds to a lead angle in the intermediateposition m corresponding to the switch angle M, that is, a switch leadangle, and “DegE” corresponds to a lead angle in the discharge side endportion of the spiral groove corresponding to the winding end angle E,that is, a winding end lead angle. For example, it is assumed that thewinding start lead angle DegS is set to 50 degrees, the switch leadangle DegM is set to 70 degrees, and the winding end lead angle DegE isset to 80 degrees. In this case, the lead angle is monotonicallyincreased by 20 degrees in a comparatively steep manner from the windingstart angle 0 to the switch angle M. On the other hand, the lead angleis monotonically increased by 10 degrees in a comparatively slow mannerfrom the switch angle M to the winding end angle E.

A total of lead obtained by turning around the axis of each screw rotor16 from the winding start angle 0 to the winding end angle E can bedetermined as an entire length L in the axial direction of the screwrotor 16 on the basis of the lead angle change function θ(x) obtained bycombining the first change function θ1(x) and the second change functionθ2(x). In other words, as shown in FIG. 2B, an axial length in the firstrange of the screw rotor 16, that is, a first axial length (an intakeside axial length) L1 is determined on the basis of the first changefunction θ1(x) corresponding to the angle range from the winding startangle 0 to the switch angle M. Further, an axial length in the secondrange of the screw rotor 16, that is, a second axial length (a dischargeside axial length) L2 is determined on the basis of the second changefunction θ2(x) corresponding to the angle range from the switch angle Mto the winding end angle E. Further, the total of both the axial lengthsL1 and L2 is determined as the entire length L in the axial direction ofthe screw rotor 16.

The first change function θ1(x), the second change function θ2(x) andthe entire length L (=L1+L2) in the axial direction of the screw rotor16 obtained on the basis of the change functions θ1(x) and θ2(x) can beexpressed by the following expressions.

First, the first change function θ1(x) corresponding to the angle range(0<x<M) from the winding start angle 0 to the switch angle M can beexpressed by the following expression (1), and a constant k1 in theexpression (1) can be expressed by the following expression (2). In thiscase, r in the expression (2) corresponds to the radius of the pitchcircle of the screw rotor 16.

θ1(x)=DegS+k1·x  (1)

k1=(DegM−DegS)/(2πr·M)  (2)

It is assumed that the winding start lead angle DegS is changed to alarge value without changing the switch angle M and the switch leadangle DegM, with respect to the first change function θ1(x) shown by asolid line in FIG. 2A. In this case, a change degree of the lead angle θfrom the winding start angle 0 to the switch angle M becomes gentlerthan that in the first change function θ1(x) shown by the solid line. Inother words, the change degree of the volumetric capacity of theactuation chamber 25 from the intake side to the discharge sidedetermining a gas compression characteristic of the pump 11 becomesgentler than that of the case of the first change function θ1(x) shownby the solid line in the first range of the screw rotor 16. In contrast,it is assumed that the winding start lead angle DegS is changed to asmall value, with respect to the first change function θ1(x) shown bythe solid line in FIG. 2A. In this case, the change degree of the leadangle θ from the winding start angle 0 to the switch angle M becomessteeper than that in the first change function θ1(x) shown by the solidline. In other words, the change degree of the volumetric capacity ofthe actuation chamber 25 from the intake side to the discharge sidebecomes steeper than that of the case of the first change function θ1(x)shown by the solid line in the first range of the screw rotor 16.

On the other hand, the second change function θ2(x) corresponding to theangle range (M<x<E) from the switch angle M to the winding end angle Ecan be expressed by the following expression (3), and a constant k2 inthe expression (3) can be expressed by the following expression (4).

θ2(x)=DegM+k2·(x−M)  (3)

k2=(DegE−DegM)/(2πr·E)  (4)

It is assumed that the winding end lead angle DegE is changed to a largevalue without changing the switch lead angle DegM, with respect to thesecond change function θ2(x) shown by the solid line in FIG. 2A. In thiscase, a change degree of the lead angle θ from the switch angle M to thewinding end angle E becomes steeper than that in the second changefunction θ2(x) shown by the solid line. In other words, the changedegree of the volumetric capacity of the actuation chamber 25 from theintake side to the discharge side becomes steeper than that of the caseof the second change function θ2(x) shown by the solid line in thesecond range of the screw rotor 16. In contrast, it is assumed that thewinding end lead angle DegE is changed to a small value, with respect tothe second change function θ2(x) shown by the solid line in FIG. 2A. Inthis case, the change degree of the lead angle θ from the switch angle Mto the winding end angle E becomes gentler than that in the secondchange function θ2(x) shown by the solid line. In other words, thechange degree of the volumetric capacity of the actuation chamber 25from the intake side to the discharge side becomes gentler than that ofthe case of the second change function θ2(x) shown by the solid line inthe second range of the screw rotor 16.

Next, a description will be given of the entire length L (=L1+L2) in theaxial direction of the screw rotor 16 introduced from the lead anglechange functions θ1(x) and θ2(x) mentioned above.

The first axial length L1 in the first range corresponding to the anglerange (0<x<M) from the winding start angle 0 to the switch angle M canbe expressed by the following expression (5).

L1=1/k1·log(sin(DegS+k1·2πr·M)/Sin(DegS))  (5)

Further, the second axial length L2 in the second range corresponding tothe angle range (M<x<E) from the switch angle M to the winding end angleE can be expressed by the following expression (6).

L2=1/k2·log(sin(DegM+k2·2πr·E)/sin(DegM))  (6)

Accordingly, it is possible to determine the entire length L (=L1+L2) inthe axial direction of the screw rotor 16 on the basis of theexpressions (5) and (6) mentioned above.

Next, a description will be given of an operation of the pump 11structured as mentioned above.

When rotated by the electric motor 22, the screw rotors 16 mating witheach other are rotated together with the rotary shafts 19, and the inertgas is sucked into the pump chamber 17 from the outside via the suctionport 24. The inert gas sucked into the pump chamber 17 is transferredtoward the discharge port while being compressed within each of theactuation chambers 25 in accordance with the rotation of both the screwrotors 16, and is discharged to the outside from the interior of thepump chamber 17 via the discharge port. Accordingly, in the case thatthe pump 11 is actuated in a state in which the suction port 24 isconnected to a working room or a working container executing variousprocesses with respect to the wafer (not shown) in the semiconductormanufacturing process, a clean vacuum environment is generated withinthe working room and the working container.

On the other hand, the screw rotor 16 executes a compression operationin accordance with the following manner. In other words, the inert gassucked into the pump chamber 17 from the suction port 24 is rapidlycompressed at a time of being transferred in the actuation chamber 25 inthe first range of the screw rotor 16, because the volumetric capacitychange degree of the actuation chamber 25 is comparatively steep.Thereafter, the inert gas is slowly compressed at a time of beingtransferred in the actuation chamber 25 in the second range of the screwrotor 16, because the volumetric capacity change degree of the actuationchamber 25 is comparatively slow. Accordingly, it is possible to avoidthe matter that the rapid pressure increase is generated near thedischarge port, and it is possible to suppress the local temperatureincrease near the discharge port.

The entire length L in the axial direction of the screw rotor 16 can bedefined on the basis of the expressions (1) to (6). In the assumptionmentioned above, in the case of changing the manner in which thevolumetric capacity of the actuation chamber 25 changes from the intakeside to the discharge side determining the gas compressioncharacteristic of the pump 11 without changing the entire length L inthe axial direction, the switch lead angle DegM is changed, for example,as shown in FIG. 2A. In this case, in the example in FIG. 2A, thewinding start angle 0, the switch angle M and the winding end angle Eare not changed, and the winding start lead angle DegS and the windingend lead angle DegE are not changed. In other words, in the case thatthe switch lead angle DegM is changed, for example, to a valueDegM′smaller than a value in the lead angle change function θ1(x) shownby the solid line in FIG. 2A, the first change function θ1(x) expressesthe gentler change degree of the lead angle θ, as shown by a one-dotchain line in FIG. 2A, and the second change function θ2(x) expressesthe steeper change degree of the lead angle θ. In this case, as shown bya one-dot chain line in FIG. 2B, the entire length L (=L1′+L2′) in theaxial direction of the screw rotor 16 becomes equal to the entire lengthL (=L1+L2) in the axial direction before changing the switch lead angleDegM. Further, in the case that the switch lead angle DegM is changed,for example, to a value DegM″ larger than the value in the lead anglechange function θ1(x) shown by the solid line in FIG. 2A, the firstchange function θ1(x) expresses the steeper change degree of the leadangle θ, as shown by a two-dot chain line in FIG. 2A, and the secondchange function θ2(x) expresses the gentler change degree of the leadangle. In this case, as shown by a two-dot chain line in FIG. 2B, theentire length L (=L1″+L2″) in the axial direction of the screw rotor 16becomes equal to the entire length L (=L1+L2) in the axial directionbefore changing the switch lead angle DegM. As mentioned above, if thelead angle change function θ1(x) is constructed by the combination of aplurality of change functions θ1(x) and θ2(x) having different manner ofchanging, the compression characteristic of the pump 11 can be changedby changing the manner in which the lead angle θ changes from thewinding start angle 0 to the winding end angle E, even in the case thatthere is any circumstance by which the entire length L in the axialdirection of the screw rotor 16 cannot be changed.

On the other hand, in the case that the entire length L in the axialdirection of the screw rotor 16 is changed without changing the firstchange function θ1(x) and the second change function θ2(x), the switchangle M is changed, for example, as shown in FIG. 3A. In this case, inthe example, shown in FIG. 3A, the winding start angle 0 and the windingend angle E are not changed, and the winding start lead angle DegS isnot changed. In other words, in the case that the switch angle M ischanged to a small value M′, for example, as shown by a one-dot chainline in FIG. 3A, the switch lead angle DegM and the winding end leadangle DegE become smaller in a state in which the change degrees of thelead angle θ respectively expressed by the first and second changefunctions θ1(x) and θ2(x) are not changed. As a result, in this case, asshown by a one-dot chain line in FIG. 3B, the entire length L in theaxial direction of the screw rotor 16 is changed to a larger value L′.Further, in the case that the switch angle M is changed to a largervalue M″, for example, as shown by a two-dot chain line in FIG. 3A, theswitch lead angle DegM and the winding end lead angle DegE become largerin a state in which the change degrees of the lead angle θ respectivelyexpressed by the first and second change functions θ1(x) and θ2(x) arenot changed. As a result, in this case, the entire length L in the axialdirection of the screw rotor 16 is changed to a smaller value L″, asshown by a two-dot chain line in FIG. 3B. As mentioned above, it ispossible to arbitrarily change the entire length L in the axialdirection of the screw rotor 16 by changing the switch angle M withoutchanging a plurality of change functions θ1(x) and θ2(x) constitutingthe lead angle change function θ1(x).

The embodiment mentioned above has the following advantages.

(1) In the present embodiment, the change of the lead angle θ in thescrew rotors 16 is expressed by the lead angle change function θ1(x)obtained by combining a plurality of change functions θ1(x) and θ2(x)having different manners of changing, from the winding start angle 0 tothe winding end angle E. Accordingly, it is possible to arbitrarily setthe manner in which the lead angle θ changes in accordance with themanner of combining a plurality of change functions θ1(x) and θ2(x).Accordingly, it is possible to arbitrarily set the compressioncharacteristic (the manner in which the volumetric capacity of theactuation chamber 25 changes) introduced by the manner in which the leadangle θ changes on the basis of a plurality of combined change functionsθ1(x) and θ2(x), in a relation with the entire length L in the axialdirection of the screw rotors 16, and it is possible to set such that acompression efficiency becomes optimum in correspondence to the kind ofthe inert gas (the fluid) to be compressed.

(2) The change degree of the lead angle θ is gentler in the second rangein each screw rotor 16 than in the first range. In other words, thevolumetric capacity change degree of the actuation chamber 25determining the compression characteristic of the pump 11 becomesgentler in the second range in each screw rotor 16 than in the firstrange. Therefore, the volumetric capacity change degree of the actuationchamber 25 becomes gentle near the discharge port of the pump 11 at thetime of the operation of the pump. Accordingly, it is possible toreliably avoid the steep pressure increase near the discharge port andthe local temperature increase caused by the steep pressure increase.

(3) Each of the first and second change functions θ1(x) and θ2(x)constituting the lead angle change function θ(x) is constituted by themonotone changing function of gradually increasing the lead angle θ inaccordance with change of the rotation angle x from the winding startangle 0 to the winding end angle E. Accordingly, the lead in the screwrotor 16 is decreased from the winding start angle 0 toward the windingend angle E. Therefore, in the case that a pair of the screw rotors 16are rotated within the pump chamber 17 while being mated with eachother, the rotation load of the screw rotors 16 becomes small, and it ispossible to achieve an improved compression operation of the pump 11.

(4) There is a case that it is required to change the compressioncharacteristic of the pump 11 (the manner in which the volumetriccapacity of the actuation chamber 25 changes) in correspondence to thekind of the inert gas to be compressed in the pump 11. In the casementioned above, in the present embodiment, the switch lead angle DegMis changed at the switch angle M corresponding to the intermediateposition m in the axial direction of the screw rotor 16. As a result, itis possible to easily change the compression characteristic of the pump11 without changing the entire length L in the axial direction of thescrew rotor 16 accommodated within the pump chamber 17 having aconstraint in space, and it is possible to compress and transfer thevarious inert gases at an optimum compression efficiency.

(5) There is a case that it is required to change the entire length L inthe axial direction of the screw rotor 16 without changing thecompression characteristic of the pump 11 (the manner in which thevolumetric capacity of the actuation chamber 25 changes) in the case ofchanging the volumetric capacity of the pump chamber 17 or the like. Inthe case mentioned above, in the present embodiment, the rotation anglex coming to a boundary where two change functions θ1(x) and θ2(x) areswitched, that is, the switch angle M is changed. In this case, theswitch lead angle DegM is also changed in accordance with the change ofthe switch angle M. As a result, it is possible to easily change theentire length L in the axial direction of the screw rotor 16 withoutchanging the compression characteristic of the pump.

In this case, the embodiment mentioned above may be changed as follows.

The fluid transferred while being compressed within the pump chamber 17in accordance with the rotation of the screw rotors 16 may beconstituted by a gas other than the inert gas (F₂ gas or the like), forexample, a cooling medium gas, or may be constituted by a liquid such asa working fluid or the like. Further, the screw pump in accordance withthe present invention may be applied to other pumps than the vacuumpump.

A plurality of change functions θ1(x) and θ2(x) combined forconstructing the lead angle change function θ1(x) are not limited to themonotone increasing function, but may be constituted by a quadraticfunction, an nth degree function, an exponential function or the like.

The number of the change functions θ1(x) and θ2(x) combined forconstructing the lead angle change function θ(x) is not limited to twobut may be set to three or more as far as it is a plural number.

The change functions θ1(x) and θ2(x) combined for constructing the leadangle change function θ(x) may be structured such that the first changefunction θ1(x) expresses the change of the lead angle θ by the gentlerchange degree than that of the second change function θ2(x), as isdifferent from that shown by the solid line in FIG. 2A.

In the case that a plurality of functions combined for constructing thelead angle change function θ(x) are constituted, for example, by thecombination of two functions, the structure may be made such that onefunction is constituted by a change function indicating a state in whichthe lead angle θ is continuously changed, and the other function isconstituted by a function indicating a state in which the lead angle θis not continuously changed. In other words, it is preferable that thescrew rotor 16 has at least a part of the portion where the lead angle θis continuously changed from the leading end (the intake side endportion) of the spiral groove (the helix) to the trailing end (thedischarge side end portion).

1. A screw gear having a portion in which a lead angle is continuously changed from a leading end to a trailing end of a spiral groove, wherein, in the case of expressing, as a lead angle change function, a change of said lead angle from a winding start angle serving as a rotation angle corresponding to the leading end of said spiral groove to a winding end angle serving as a rotation angle corresponding to a trailing end of said spiral groove with respect to a rotation angle of said screw gear around an axis, said lead angle change function is constituted by a combination of a plurality of change functions having different manners of changing.
 2. The screw gear according to claim 1, wherein a predetermined rotation angle between said winding start angle and said winding end angle is set as a switch angle, wherein said lead angle change function includes a first change function corresponding to an angle range from said winding start angle to said switch angle, and a second change function corresponding to an angle range from said switch angle to said winding end angle, and wherein said second change function expresses the change of the lead angle by a gentler change degree than said first change function.
 3. The screw gear according to claim 1, wherein each of said change functions constituting said lead angle change function is constituted by a monotone changing function gradually increasing the lead angle in accordance with change of said rotation angle from said winding start angle to said winding end angle.
 4. The screw gear according to claim 1, wherein the following expressions are satisfied: θ1(x)=DegS+k1·x (in this case 0<x<M) k1=(DegM−DegS)/(2πr·M) θ2(x)=DegM+k2·(x−M) (in this case M<x<E) k2=(DegE−DegM)/(2πr·E) L1=1/k1·log(sin(DegS+k1·2πr·M)/Sin(DegS)) L2=1/k2·log(sin(DegM+k2·2πr·E)/Sin(DegM)) L=L1+L2 in which x: rotation angle, 0: winding start angle, E: winding end angle, M: switch angle corresponding to a rotation angle x set between the winding start angle 0 and the winding end angle E, DegS: winding start lead angle corresponding to a lead angle at the winding start angle 0, DegE: winding end lead angle corresponding to a lead angle at the winding end angle E, DegM: switch lead angle corresponding to a lead angle at the switch angle M, θ1(x): change function indicating a change of the lead angle in an angle range (0<x<M) from the winding start angle 0 to the switch angle M, θ2(x): change function indicating a change of the lead angle in an angle range (M<x<E) from the switch angle M to the winding end angle E, k1 and k2: constants, r: radius of a pitch circle of the screw gear, L: entire length in an axial direction of the screw gear, L1: length in an axial direction of the screw gear corresponding to the angle range 0<x<M, L2: length in an axial direction of the screw gear corresponding to the angle range M<x<E.
 5. (canceled)
 6. A screw pump apparatus comprising: a pair of screw gears mating with each other; and a pump chamber accommodating the screw gears, wherein the screw gears rotate while mating with each other, whereby a fluid sucked into the pump chamber is transferred in an axial direction of the screw gears while being compressed within said pump chamber, wherein an actuation chamber for compressing the fluid is defined between adjacent thread ridge portions in the axial direction of each screw gear, wherein each screw gear has a portion in which a lead angle is continuously changed from a leading end to a trailing end of a spiral groove, and wherein, in the case of expressing, as a lead angle change function, a change of said lead angle from a winding start angle serving as a rotation angle corresponding to the leading end of said spiral groove to a winding end angle serving as a rotation angle corresponding to a trailing end of said spiral groove with respect to a rotation angle of each screw gear around an axis, said lead angle change function is constituted by a combination of a plurality of change functions having different manners of changing. 